This proposal draws a comparison between a neurobehavioral emotional system described in practically all species, and a higher order factor that is one major component of the structure of personality. The former is the Behavioral Facilitation System (BFS), and the latter is Positive Emotionality (PE), similar to extraversion. Behaviorally, the two systems are described quite similarly: both social and sexual interaction by signals of reward. The neurobiology of the BFS has been studied extensively in animals: it is based on the ascending dopamine (DA) projections originating in the ventral tegmental area and terminating in the limbic striatum and forebrain, and neocortex. Because the neurobiology associated with PE is unknown, analogy between the animal BFS and human PE constructs has not progressed. The preliminary and proposed studies described herein attempt to assess DA functioning in normal twins naturally distributed across the dimension of PE, as measured by Tellegen's (1982) Multidimensional Personality Questionnaire (MPQ). A strictly correlational nature of the study is circumvented in part by the use of a challenge protocol employing a direct DA D2 receptor agonist. DA functioning is assessed via the action of a DA agonist on prolactin (PRL) secretion, the rate of spontaneous eye blinking, and spatial delayed-response performance, all variables being strongly influenced by DA D2 receptor action. Preliminary studies demonstrated an extremely strong correlation between PE and the DA agonist's effects on these variables. Three major aims of the proposed studies are: 1) To determine, by sex, the range of three DA agonist doses that is most sensitive to individual differences in DA agonist effects on the DA indices; 2) To determine the degree of genetic influence on (a) each DA variable, and (b) pairs of DA and personality variables via use of a twin population in a dose-response design; and 3) To determine the relationship between a host of personality inventories (having convergent and divergent constructs to the BFS or PE constructs) and DA reactivity to the agonist. The best set of DA-related personality subdomains will be identified in order to revise the MPQ PE scale for selection of research subjects in future studies. Also, major hypotheses concerning the interaction of personality traits will be tested with the DA variables being the predicted criterion, which is the first such analysis where biology is incorporated. The potential significance of these studies for mental health issues lies in the detailed explication of a personality-neurobehavioral system that, when incorporating genetic vulnerabilities, may contribute to (a) abuse of substances that depend on DA function for their psychogenic effects, including alcohol, and (b) certain forms of affective disorder that are DA-related, most particularly bipolar affective disorders.